This invention relates to a method of projection-welding bolts and is utilized, for example, in a field where bolts are welded to the floor panel of an automobile.
In the case of welding bolts to a part in the form of a steel sheet, as shown in FIG. 12, a flange F integral with a bolt B is formed with a plurality of projections (resembling warts) P, which projections will be fused in welding. In this case, generally a steel sheet part is placed on the fixed electrode and a bolt is pressed against the steel sheet part by the movable electrode. In this state, electric current is passed to effect fusion of the projections.
With the prior art as described above, since only the projections are fused, the fused area available is no more than the total area of the projections; therefore, a high welding strength resulting from a large fused area cannot be attained. Further, because of the fusion solely of the projections, the flange projects from the steel sheet part by the amount corresponding to its thickness, sometimes resulting in the part occupying an excessively large installation space.